A perfect static type checker would distinguish between valid and invalid programs in all cases. In practice, static type checkers are conservative and occasionally reject valid programs. Gradual typing gives you a way out in these situations -- just like how macros in C can provide type-generic functionality.

A sufficiently rich type system (with tuples, union types, traits, generics, higher-kinded types, etc) can lessen the need for a dynamic escape hatch. In the end though, the occasional use of dynamic typing can be easier to reason about.